Charge-Transfer Mechanism for Cytochrome <i>c</i> Adsorbed on Nanometer Thick Films. Distinguishing Frictional Control from Conformational Gating

Using nanometer thick tunneling barriers with specifically attached cytochrome <i>c</i>, the electron-transfer rate constant was studied as a function of the SAM composition (alkane versus terthiophene), the ω-terminating group type (pyridine, imidazole, nitrile), and the solution viscosity. At large electrode−reactant separations, the pyridine terminated alkanethiols exhibit an exponential decline of the rate constant with increasing electron-transfer distance. At short separations, a plateau behavior, analogous to systems involving −COOH terminal groups to which cytochrome <i>c</i> can be attached electrostatically, is observed. The dependence of the rate constant in the plateau region on system properties is investigated. The rate constant is insensitive to the mode of attachment to the surface but displays a significant viscosity dependence, change with spacer composition (alkane versus terthiophene), and nature of the solvent (H<sub>2</sub>O versus D<sub>2</sub>O). Based on these findings and others, the conclusion is drawn that the charge-transfer rate constant at short distance is determined by polarization relaxation processes in the structure, rather than the electron tunneling probability or large-amplitude conformational rearrangement (gating). The transition in reaction mechanism with distance reflects a gradual transition between the tunneling and frictional mechanisms. This conclusion is consistent with data from a number of other sources as well.